Luminescent screen



Patented Oct. 28, 1941 Frank E. 'Swindells,

Patterson Screen Company,

Towanda, Pa., assignorto' The .Towanda, Pa., a

corporation of Pennsylvania No Drawing. Application October -24, 1940,Serial No. 362,681

4- Olairns. *(Gl. 250-81) This invention relates to television apparatusand particularly to luminescent viewing screens adapted to be renderedfluorescent by cathode rays and the like.

Many'current methods of television reproduction operate at an image rateof thirty images or frames per second. At the receiving station, imagesare reproduced on a luminescent screen in television tubes, cathode raytubes, and the like. It has been determined that television reproductionat the rate of approximately fifteen frames or images per second hasmany inherent advantages over the thirty frames per second system. Thepresent invention pertains to a luminescent viewing "screen intended foruse particularly with fifteen images per second television reproductionapparatus.

It is the object of my invention to provide a luminescent viewing screenfor television apparatus which will reproduce images at the rate offifteen per second without producing the sensation of flicker, as oneimage replaces another on the viewing screen.

It is the further object of this invention to provide a luminescentscreen having substantially white fluorescence, and an after-glow of substantially one-fifteenth second duration.

It is another object of this invention to provide a luminescent screenof any desired tint or shade of fluorescence together with the specificextent or duration of after-glow heretofore mentioned.

It is another object of this invention to provide a luminescent screenfor fifteen frames per second television which has an after-glow ofadequate luminosity for one-fifteenth second duration, the purpose ofthe said after-glow being to eliminate the sensation of flicker in thereproduction of television motion pictures.

Other objects and advantages of this invention will be readily apparentfrom the following description of the preferred embodiment of theinvention described below.

Heretofore, many different materials have been proposed for fluorescentscreens in television and cathode ray tubes. Materials commonly usedinclude zinc sulfide, zinc cadmium sulfide, zinc silicate, zincberyllium silicate, and others. In the thirty images or frames persecond television apparatus utilizing such materials, no objectionablesensation of flicker is apparent. However, the use of such fluorescentmaterials in the fifteen frames per second system results in a veryobjectionable sensation of flicker as each image replaces another. Ihave discovered that a viewing screen having an after-glowcharacteristic providing luminescence of adequate intensity forapproximately one-fifteenth second duration effectively eliminates thesensation of flicker in the fifteen frames per second system. Testsindicate that the usual fluorescent materials provide either too muchafter-glow resulting in blurred images, or too short or too weakafter-glow resulting in objectionable flicker.

My investigations show that cadmium silicate fluorescent material has adesired after-glow of effective luminosity up to one-fifteenth secondduration. Such material is preferably manganese-activated, as is wellknown in the fluorescent material art. However, cadmium silicate has a.red or pink luminescence which is not particularly desirable intelevision screens. I have found that this material may be mechanicallymixed with other fluorescent materials to modify the objectionable pinkor red fluorescence characteristic, without seriously impairing thedesired after-glow characteristic. For example, tests show that a screenprepared from a mechanical mixture of 55 parts manganese-activatedcadmium silicate, 30 parts silver activated zinc sulfide, and 15 parts(manganese-activated) zinc silicate provides substantially whitefluorescence and flickerless reproduction of images in fifteen framesper second television apparatus. The after-glow is of adequateluminosity up to onefifteenth second duration, and thereafter negligibleor insufficient to cause blurred reproduction.

The proportions of the materials mixed with the cadmium silicate may bevaried to produce fluorescence of warm white, cold White, daylightwhite, or various desired tints and shades. Varying the cadmium silicatein the range of 53 to 61% by weight, the zinc sulfide from 26% to 31% byweight, and/or the zinc silicate from 12.5 to 18% by weight, makes widevariation in the tint or shade of fluorescence of the mixture. As theresult of my research, I have found that the after-glow of availablecadmium silicate is such that the fluorescent material mixture shouldcontain not substantially less than 50% cadmium silicate, by weight, inorder to provide an afterglow of sufiicient intensity to eliminate thesensation of flicker in fifteen frames per second televisionreproduction. The use of less than 50% cadmium silicate in thefluorescent material mixture is indicated where the available cadmiumsilicate in smaller quantity provides an afterglow of adequate intensityfor one-fifteenth second duration to eliminate the sensation of flicker.Higher percentages of cadmium silicate, i. e.,

as somewhat greater than 61% by weight, provide greater intensity ofafter-glow, but detract from the whiteness of fluorescence that isusually de sired in television reproduction. Other materials may beadded to or substituted for the zinc sulfide and zinc silicatecomponents of the fluorescent material mixture in order to obtainfluorescence of a particular desired tint or shade.

Although I have described and claimed my invention with particularreference to reproduction at the rate of fifteen images per second, itis readily apparent that the desirable advantages of this invention arealso obtained at reproduction rates somewhat above and/or below fifteenimages per second. Therefore, it is my intent in reciting approximatelyfifteen images per second, and in reciting an after-glow duration of theorder of onefifteenth second, or substantially one-fifteenth second, toinclude therein those rates either above or below which approximate thefigures specified.

What I claim is:

l. A luminescent screen material for use in fifteen images per secondtelevision reproduction apparatus comprising a mixture of 55% by weightmanganese-activated cadmium silicate, 30% by weight silver-activatedzinc sulfide, and 15% by Weight manganese-activated zinc silicate, toprovide a screen of substantially white fluorescence having anafter-glow of the order of one-fifteenth second duration for eliminatingthe sensation of flicker as one image replaces another on said screen.

2. In television apparatus for reproducing images at the rate ofapproximately fifteen per second, a Viewing screen of luminescentmaterial activated by cathode rays, said material comprisingmanganese-activated cadmium silicate present at least in an amount onthe order of fifty per cent by weight, the luminescent material havingan after-glow the duration of which is of the order of one-fifteenthsecond, when activated by the cathode rays, for eliminating thesensation of flicker as one image replaces another.

3. In television apparatus for reproducing images at the rate ofapproximately fifteen per second, a viewing screen of luminescentmaterial activated by cathode rays, said material comprisingmanganese-activated cadmium silicate present at least in an amount onthe order of fifty per cent by Weight, mixed with other luminescentsubstances having a color of fluorescence different from that of saidcadmium silicate, the luminescent material having an after-glow theduration of which is of the order of one-fifteenth second, whenactivated by the cathode rays, for eliminating the sensation of flickeras one image replaces another.

4. In television apparatus for reproducing images at the rate ofapproximately fifteen per second, a viewing screen of luminescentmaterial activated by cathode rays, said material comprisingmanganese-activated cadmium silicate, silveractivated zinc sulfide andmanganese-activated zinc silicate, said cadmium silicate being presentat least in an amount on the order of fifty per cent by weight, theluminescent material having an after-glow the duration of which is ofthe order of one-fifteenth second, when activated by the cathode rays,for eliminating the sensation of flicker as one image replaces another.

FRANK E. SWINDELLS.

